Creme fraiche with capers and mustard
3-4 tablespoons of creme fraiche (or half creme fraiche if you prefer)
Or you could use half greek yoghourt/creme fraiche
A few squeezes of lemon - to taste
Chopped capers - about 1 tablespoon
Black pepper - freshly ground, to taste
1/4 tsp of mustard
Bit of horseradish (if you happen to have some)
1 spring onion, chopped (optional)
This is quite a rich sauce, but is perfect with roasted salmon and steamed potatoes.